<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    * {
      margin: 0;
      padding: 0;
    }

    li {
      list-style: none;
    }

    a {
      text-decoration: none;
      font-size: 16px;
    }

    .nav {
      margin: 100px;
    }

    .nav>li {
      position: relative;
      float: left;
      width: 80px;
      height: 41px;
      text-align: center;
    }

    .nav li a {
      display: block;
      width: 100%;
      height: 100%;
      color: #333;
      line-height: 41px;
    }

    .nav>li>a:hover {
      background-color: #eee;
    }

    .nav ul {
      display: none;
      position: absolute;
      top: 35px;
      left: 0;
      width: 100%;
      border-left: 1px solid #fecc5b;
      border-right: 1px solid #fecc5b;
    }

    .nav ul li {
      border-bottom: 1px solid #fecc5b;
    }

    .nav ul li a:hover {
      background-color: #fff5da;
    }
  </style>
</head>

<body>
  <ul class="nav">
    <li>
      <a href="#">微博</a>
      <ul>
        <li>
          <a href="#">私信</a>
        </li>
        <li>
          <a href="#">评论</a>
        </li>
        <li>
          <a href="#">@我</a>
        </li>
      </ul>
    </li>
    <li>
      <a href="#">微博</a>
      <ul>
        <li>
          <a href="#">私信</a>
        </li>
        <li>
          <a href="#">评论</a>
        </li>
        <li>
          <a href="#">@我</a>
        </li>
      </ul>
    </li>
    <li>
      <a href="#">微博</a>
      <ul>
        <li>
          <a href="#">私信</a>
        </li>
        <li>
          <a href="#">评论</a>
        </li>
        <li>
          <a href="#">@我</a>
        </li>
      </ul>
    </li>
    <li>
      <a href="#">微博</a>
      <ul>
        <li>
          <a href="#">私信</a>
        </li>
        <li>
          <a href="#">评论</a>
        </li>
        <li>
          <a href="#">@我</a>
        </li>
      </ul>
    </li>
  </ul>

  <script>
    var nav = document.querySelector('.nav');
    var lis = nav.children; // 得到4个小li
    // 循环注册事件
    for (var i = 0; i < lis.length; i++) {
      lis[i].onmouseover = function () {
        this.children[1].style.display = 'block';
      }
      lis[i].onmouseout = function () {
        this.children[1].style.display = 'none';
      }
    }
  </script>
</body>

</html>